The International Red Cross said a third of the country's 9 million people may need emergency aid. Red Cross volunteers on the ground have been providing first aid to victims but ran out of supplies Wednesday night.

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"The limited medical supplies that the Red Cross had in Haiti before the earthquake have now been distributed and exhausted," American Red Cross spokesman Eric Porterfield told ABC News. Thirty relief workers left Santo Domingo at 4 a.m. local time today to travel to Haiti with additional medical supplies.

Haitian Red Cross spokesman Pericles Jean-Baptiste told Reuters that "There are too many people who need help," and former president Bill Clinton said that some places don't even have aspirin.

A doctor with the aid organization Partners in Health tweeted that it had been "getting overwhelmed in terms of wounded coming from [Port-au-Prince]. Will need orthopedic support as well."

Partners in Health has set up mobile clinics in Port-au-Prince and has hospitals about two hours outside the city. On its Web site, the organization is asking for volunteer "surgeons (especially trauma/orthopedic surgeons), ER doctors and nurses, and full surgical teams (including anesthesiologists, scrub and post-op nurses and nurse anesthetists)."

The Caribbean nation already has "some of the worst health indicators in the world," Besser said, and has a limited ability to absorb this kind of a catastrophe. Haiti is the poorest country in the Western Hemisphere, and the population of 9 million already faces high rates of tuberculosis, HIV/AIDS, viral and respiratory disease.

According to the U.S. Agency for International Development, Haiti has the highest per capita tuberculosis burden in the Latin American and Caribbean region. After HIV/AIDS, TB is the country's greatest infectious cause of mortality.

U.S. Government Mobilizes Response After Haiti's Earthquake

Besser said that medical aid would be one of the most important parts of the aid effort, because Haiti's health care capacity was already very limited before the quake. The U.S. is preparing the hospital ship the USS Comfort for possible deployment to the region.

"Access to quality care didn't exist before this earthquake," he said. "It doesn't exist now, so what's important is arranging evacuations and setting up hospitals. ... There's not a lot of time to save the most critically injured."

Besser said that the first 36 hours after a disaster is the most critical period, but that during these first few days
"communities tend to have to take care of themselves."

The Associated Press is reporting that Haitians are doing just that -- using pick-up trucks as makeshift ambulances and doors as stretchers to transport victims.

Haiti's proximity to the United States makes it easier to get help there quickly, as opposed to the long delays in getting aid across the Pacific Ocean after the 2004 Indian Ocean earthquake, which left about 23,000 dead.

The ABC News medical unit spoke to a number of recovery agencies, and Besser said all are asking, "How do we get into Haiti? How do we provide those resources? Each part of the U.S. government is rostering teams."

The U.S. government is also focused on providing health and humanitarian aid.

President Obama said Wednesday that his administration would make a "swift, coordinated and aggressive effort to save lives," focusing on delivering "the humanitarian relief, food, water and medicine that Haitians will need in the coming days."

One of the urgent priorities was to mobilize more "rescue, medical equipment and emergency personnel" in the coming days.

The U.S. has prepared the hospital ship the USS Comfort for possible deployment to the region.

Rajiv Shah, an administrator with the U.S. Agency for International Development, said Wednesday that his organization has been in contact with its mission director on the ground in Haiti, and that it has "deployed an aggressive and coordinated response."

Health Consequences After Earthquake

After the initial rescue period, Besser said additional health consequences from the destroyed infrastructure can emerge. Areas of the country may lose water, sanitation and access to clean food, and all of this leads to additional health problems.

"Water is critical," he said. "An adult needs at least five liters of water a day. Dirty water is better than no water at all."

According to the World Health Organization, no city in Haiti has a public sewage system, and less than half of the population has access to drinking water services.

This kind of a crisis can raise the risk of outbreaks of infectious disease, particularly because so many people are left homeless, and refugee camps pose a risk for communicable diseases. The most prevalent infectious diseases are bacterial and protozoal diarrhea, hepatitis A and E, typhoid fever, dengue fever, malaria and leptospirosis.

According to UNICEF, only about 50 percent of children in Haiti received vaccinations for diseases such as DPT (diphtheria, pertussis and tetanus), polio and measles in 2007.

But Besser noted that large outbreaks of infectious disease are relatively rare, and immediate efforts will be focused on those who can "use the care and need the assistance."

He said the Centers for Disease Control and Prevention has approximately 1,000 people available, including environmental engineers, to address water and sanitation problems, and teams to "pick up on some of the public health programs."

Besser said public fears over dead bodies during a disaster are generally unfounded.

"That's one of the biggest myths," he said. "Everyone is most concerned about dead bodies. You're more likely to get a disease from a living person than a dead body."